talk to andy @ AMR Engineering . it's
http://www.amrengineering.com for his contact info. he'll custom valve his coilovers for whatever spring rate you like, and as long as you stay off the track, they're warrantied for life!. he's been building coilovers for several years now, but most of his sales are honda stuff. he makes an S13 coilover, though. they're just a hair under $1,000, and have aluminum bodies. rubber or poly top mounts i think, though, no pillow ball, but he'll custom machine pillow balls for you for super cheap, probably like $50ish.
BTW, having such a low rear spring rate is like following the lemming in front of you off a cliff. yes, it's stiffer than stock, but the rear motion ratio (~0.70), at least on an S13, is way lower than the front motion ratio (~0.91), so the actual wheel rate of 7/5 actually turns out to something like 6.4/3.5. super soft rear is nice for coming out of corners, but you'll lose a lot of road-holding for it. maybe if you have an iron block, iron-headed V8 mounted right up against the radiator those would be good spring rates, though.
zerothread/331315 if you want more info on that.